I also stop more often than I used to and take a rest-area nap for an hour or so when I feel the slightest bit "zoned." And I utilize my new secret weapons against losing focus: lots and lots of water and a lot of Trident cinnamon chewing gum. My theory on the former is that it's hard to space out when you're well hydrated -- dehydration and exhaustion go hand in hand -- not to mention that having to pee every hour necessitates more stops and therefore more variation. My theory on the latter, less obvious, is that a prime cause of drifting in drivers -- at least those who aren't simply exhausted to the point of unconsciousness -- is the fact that their eyes are focusing at the same point for so long that their facial muscles freeze up, as they might during the watching of a film, only more dramatically, because there's not much need to look laterally on a freeway, particularly in the vast wastelands of western Arizona and southeastern California. Chewing gum keeps those facial muscles active and guarantees that the eyes can't stay focused on the vanishing point, but need to continually refocus.
Incidentally, I've also altered my route to L.A. Whereas I used to take Interstate 8 and then cut up on, what is it, Arizona 85 (?) from Gila Bend to Buckeye, avoiding the congestion of the Phoenix area, I now stay on Interstate 10 all the way through Chandler, Tempe, Phoenix etc. It takes longer, but there's more to see, more places to stop, and more cars to keep my attention.
Since I've started doing all this, the drive has gotten an hour to ninety minutes longer, but I don't feel as tired afterwards and stay more focused on the task at hand.